1. Field of the Invention
The present invention generally relates to a method of formation of a low resistive strap for high density trench drams.
2. Description of the Related Art
Conventional processing of trench dynamic random access memory (DRAM) structures forms a buried strap at the top of the deep trench. The strap resistance has been shown to be a significant bottleneck in trench DRAM characteristics. The conventional process causes the strap to be etched away in unwanted areas during the shallow trench isolation (STI) definition (which is defined by the AA or Active Area mask) leaving a sliver of silicon along the trench sidewalls to contact to the deep trench portion below the STI. This contributes significantly to the strap resistance variation since the resistance of the above mentioned sliver is dependent on (Deep Trench) DT-AA overlay which is limited by tool capability.
Various schemes have been proposed (sometimes with an oxide cap on top of the trench referred to as a xe2x80x98Pedestalxe2x80x99 STI) leaving the polysilicon in the trench untouched but etching only the silicon outside the trench or performing the strap recess after the active area (AA) etch with a xe2x80x98Poly Planarizedxe2x80x99 STI etch. However, neither of these schemes work with a xe2x80x98lipxe2x80x99 or near surface strap which is deemed necessary to avoid a deep strap and consequent poor array device performance. It is also to be noted that all of the schemes above do not address the increased strap resistance of a dielectric present at the strap poly silicon interface. Therefore, there is a need for a method of forming a DRAM structure with a low resistance strap.
It is, therefore, an object of the present invention to provide a structure and method for a dynamic random access device which includes a substrate having a trench, a conductor in the trench, a transistor adjacent to the trench and a conductive strap electrically connecting the conductor and the transistor, wherein the strap has a lower resistance than the conductor. The conductor comprises a first material having a first resistance and the strap comprises a second material different than the first material having a second resistance, wherein the second resistance is lower than the first resistance. The strap comprises at least two electrically connected strap conductors. A first strap conductor is adjacent to the conductor and a second strap conductor is adjacent the transistor and the first strap conductor has an improved interface with the conductor. The strap comprises a lip strap, and the first strap forms an L-shape. The first strap is contiguous with a corner of the trench. The strap comprises a first strap conductor and a second strap conductor and the conductor is contiguous with the first strap conductor and the second strap conductor wherein the second strap conductor and the conductor form an L-shape.
The invention further comprises a method of forming a dynamic random access memory device which includes forming a trench in a substrate, placing a conductor in the trench, placing a transistor adjacent to the trench forming a conductive strap which electrically connects the conductor and the transistor, wherein the strap comprises a plurality of strap conductors and the strap has a lower resistance than the conductor.